[patch/cla] Addition of array.unique to Dojo's array functions.

I think a function to return the unique values of an array, analagous to array_unique in PHP, would be a useful addition to Dojo's excellent array additions. It's a function I find regularly useful in other languages (and have just found a need for it in Javascript), and was surprised not to find it in Dojo. Code like

At this point, I think the recommendation would be to just use an ES6 Set or WeakSet? if this capability is really needed. If someone wants to backport to 1.x, we'll consider a patch. In Dojo 2, we've added a shim for Map, but we haven't added Set, ​https://github.com/dojo/core/tree/master/src